Another place to check your zone is at your county extension office -- the agent should know your zone or will know where to find out (our agent is a grain 'n' livestock guy -- LOTS of fun to ask plant questions!). We also have a state extension horticulturist, located at our land grant university. He has email and is pretty good about answering questions.
A couple more thoughts about zones:
The zones only indicate how cold an area was likely to get in the winter; nothing else was measured. Which means that while I live in zone 3b - 4a here in northcentral Montana, the climate is NOTHING like the same zone in say, Maine. So, you need to know about your weather, soils, and microclimates in addition to your zone to make fully informed choices of what to plant where. Hopefully, your extension agent can help you, or at least point you in the direction of your state's extension publications.
Not every winter gets as cold as it has historically, so while some plants needing a warmer zone can get along fine, a regular winter will damage them or do them in.
There are now heat zones, too, but it's hard to get the information for specific plants. This info is really good for more southern areas than mine.
Check out environmental indicators to indicate what the soil and air are doing, and you can plant by them. For example, potatoes are best planted (around here!) when the dandelions are blooming in OPEN places -- not in the shelter of a building where the microclimate is different. This indicates the ground has warmed up enough for them to grow.
Some other ones are when the lilacs bud, bloom, and leaf; what weeds are germinating (different ones germinate at different soil temperatures); and anything else you might notice.
TT, I live in zone 8 where the average last frost date is around March 20 and traditionally people always planted by the Almanac which said plant potatoes on the old 12 days which is the first 12 days of the new year (I don't know why they call it old 12 days). I personally have found from experience that its useless to try and plant too early, they will only get knocked back by a freeze and a later planting will out perform. You can plant potatoes most anytime the nights are longer than the days for best production, I have planted them during the summer but production was scant. If you want really good tasting potatoes, plant in new ground and keep the soil on the acid side to prevent scab and eliminate chemicals. Be sure and get certified seed when you plant, this is very important.
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